Nuclear fusion is the process by which multiple like-charged atomic nuclei join together to form a heavier nucleus. Nuclear fusion is usually accompanied by the release or absorption of energy. Large scale fusion processes, involving many atoms fusing at once, occur in matter that is in a plasma state. For example, the fusion of two nuclei with lower mass than iron generally releases energy, while the fusion of nuclei heavier than iron absorbs energy. In the simplest case of hydrogen fusion, two protons have to be brought close enough for their mutual electric repulsion to be overcome by the nuclear force. The fusion of the protons results in the release of energy.
Research into controlled fusion, with the aim of producing fusion power for the production of electricity, has been conducted for over 50 years. It takes considerable energy to force nuclei to fuse, even those of the lightest element, hydrogen. This is because all nuclei have a positive charge due to their protons and since like charges repel, nuclei strongly resist being put too close together. When accelerated to high speeds, e.g., heated to thermonuclear temperatures, the nuclei can overcome this electromagnetic repulsion and get close enough for the attractive nuclear force to be sufficiently strong to achieve fusion. The fusion of lighter nuclei, which creates a heavier nucleus and a free neutron, generally releases more energy than it takes to force the nuclei together. This is an exothermic process that can produce self-sustaining reactions. At short distances, the attractive nuclear force is stronger than the repulsive electrostatic force. Therefore, if two nuclei are brought sufficiently close to each other, the likelihood that they will fuse together increases significantly. However, the main technical difficulty in achieving fusion is getting the nuclei close enough to fuse.
What is needed is a more repeatable and controllable process of achieving fusion.